This invention relates to communication networks in general and more particularly to a method and apparatus for providing optimal utilization of the transmission bandwidth when a station is capable of transmitting multiple frames onto the ring before releasing the token. The invention uses the token holding timer (THT) as a time base to calculate the maximum number of complete frames that can be transmitted within that time.
In a token ring network, a transmit station can send frames using either a single frame or a multi-frame mode. In single frame mode, a station sends only one frame for each usable token that it acquires. In multi-frame mode, a station sends multiple frames per usable token. The maximum number of frames a station can transmit per usable token is limited by the token holding timer which is specified in the applicable standard, i.e., either the IEEE 802.5 standard for token rings or the ANSI standard X3T9.5 for fiber distributed data interface (FDDI).
One way of managing the transmission of a multi-frame transmit sequence is for the transmit station to transmit its frame queue until it gets to the last frame in the queue. If, at that time, the THT has not expired, the transmitter terminates the transmit sequence by marking the intermediate frame bit (I-bit) in the ending delimiter of the current frame as the last frame. If the transmit frame sequence is not completed at the end of the THT window, the transmit station aborts the partially transmitted last frame and releases a token.
Although this method is simple and inexpensive to implement, a major disadvantage is that if the last frame transmitted is a long frame relative to the THT window and gets aborted due to expiration of the THT, the ring bandwidth used to transmit the partial last frame is wasted. This effectively reduces the throughput of the network. This method is only applicable to the ANSI X3T9.5 protocol for FDDI. It does not apply to the IEEE 802.5 protocol for token rings because the transmitter in this protocol does not release a token after aborting a frame.
Thus, there is a need to provide a method and apparatus for determining whether or not to transmit another frame waiting in the frame queue that eleminates the transmission of frames that otherwise would be aborted by the expiration of the token holding timer, and that is applicable to both the IEEE 802.5 and FDDI protocols.